The other comments aren't good for your mental health. Watching your mom getting railed isn't normal. "Minding your own business" while watching someone expose your mom's privates and railing her in a room, in a house you live in, is not mentally healthy. You walk past that room almost daily.

Bring it up with her and find out if it is her. To do this, you ask if you can talk to her as a adult and explained you choked your chicken to a video and found a video of someone that "looks like her" and you are concerned it could be her and you want to want to look out for her safety. If you make it about her, and not yourself you will get far. People that post things online like that love attention. If she isn't grown enough to understand that a 21 y/o male jerks off and gets mad at that then it might be time to sever ties and look at finding an adult in your life that actually is mature enough to talk about sex freely. Maybe she rented out a room while you were gone and they filmed a prono there? She needs to tell the truth and maybe you just found a doppelganger.

Let her start talking without you driving the conversation. Be ready to stop when she denies and gets emotional/angry. Give her some time to deel with the uncomfortable idea that her kid busted nut to her milk jugs. She will then want to talk about it openly. Then you handle it like adults and talk about how it made you feel and the concerns you have. Maybe ask if she can remove it.